A domed city is a hypothetical structure that encloses a large urban area under a single roof. In most descriptions, the dome is airtight and pressurized, creating a habitat that can be controlled for air temperature, composition and quality, typically due to an external atmosphere that is inimical to habitation for one or more reasons. Domed cities have been a fixture of science fiction and futurology since the early 20th century, offer inspirations for potential utopias and may be situated on Earth, a moon or other planet.